1. Field of the Invention
The present invention relates to an X-ray generator such as for lithography to be used in exposure replicating such as of an internal pattern of a very large scale integrated circuit (VLSI), and more particularly to an X-ray generator including means for effectively dissipating the heat generated at an anode on radiation of X-rays.
2. Description of the Prior Art
Recently, great attention has been paid to X-ray lithography for use as exposure equipment in the process of exposure replicating such as of an internal pattern of a very large scale integrated circuit (VLSI). This is because, as the line width of the internal pattern fabrication of the VLSI is so thin as to be submicrons, the X-ray lithography is quite effective means for accurately forming such an internal pattern of the VLSI by the exposure replicating. X-rays also have medical application in addition to such exposure replicating of the internal pattern of the VLSI.
In order to achieve effective and stable generation of X-rays, excessive increase in temperature of the anode must be prevented. This is because high temperature at the anode causes thermal distortion, which results in unstable generation of X-rays. Therefore, in order to obtain intensive X-rays stably, the anode must be cooled so as to hold the local temperature difference in the anode below, for example, 500.degree. C./mm.
An article entitled "X-ray Source Technology for Microlithography" described in Semiconductor International Vol. 6, No. 9 (published in September of 1983) pp. 74-77 reports that a rotary anode or a fixed anode can be used as an X-ray source, and the rotary anode is mainly for medical application, while the fixed anode is for X-ray lithography. As the intensity of X-rays for medical application is relatively low, the temperature of heat generated at the anode is low and can be sufficiently dissipated in the vacuum to prevent excessive increase in temperature at the anode.
The article further reports that, on the contrary, X-ray lithography, using relatively intensive X-rays which cause large heating value at the anode, employe not a rotary anode but a fixed anode which is cooled by water for heat sink.
The prior art rotary anode as described above is cooled by means which permits heat generated at the anode to be dissipated from the anode in rotation directly into the vacuum. Consequently, it has a limit in its heat-dissipating effect, so that generation of intensive X-rays will cause thermal distortion, which makes it very difficult to generate X-rays in a stable condition. In case of cooling by water the rotary member or the anode in the X-ray generator which is held in a high vacuum, it requires a complex and expensive sealing mechanism, which is also quite difficult to maintain. On the other hand, in case of the fixed anode, the sealing mechanism for water cooling is simple, but the overall size becomes large. Therefore, provision of an effective and simple X-ray generator has been long desired.